Stand-Alone Fusion Implant Secured by In-Line Fixation

ABSTRACT

Stand alone fusion cage assemblies that can be secured to the adjacent vertebral bodies via an in-line approach that is substantially perpendicular to the anterior wall of the stand alone cage (or substantially parallel to the cage insertion direction).

CONTINUING DATA

This patent application is a continuation of and claims priority fromco-pending US non provisional application U.S. Ser. No. 15/402,926,filed Jan. 10, 2017, entitled “Stand-Alone Fusion Implant Secured byIn-Line Fixation” (Mazzuca et al.) (DSP5218USNP), and from USprovisional application U.S. Ser. No. 62/278,590, filed Jan. 14, 2016entitled “Stand-Alone Fusion Implant Secured by In-Line Fixation”(Mazzuca et al.) (DSP5218USPSP), the specifications of which areincorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

Stand-alone fusion cages have become popular in the field of spinalfusion surgery because they can be used without the need for posteriorfixation. In general, at least two fixation devices (such as screws ornails) are passed at an angle through the anterior wall of thestand-alone cage and into the adjacent vertebral bodies, therebysecuring the stand-alone cage to the adjacent vertebral bodies.

However, one challenge with these devices is that the need to pass thefixation device at an angle to the cage requires that the inserter alsobe disposed at an angle, thereby complicating the surgery. In somefusion cases using a stand alone cage, the surgeon must insert thefixation device into the cage at a sharp angle through sometimeschallenging approaches. This can be especially difficult for thecervical spine, as the surgeon needs to either deliver the screw downinto the inferior vertebral body but may be obstructed by the patient'schin, or deliver the screw up into the superior vertebral body but maybe obstructed by the patient's sternum.

Therefore, there is a need to provide a stand alone fusion assembly thatcan be secured to the adjacent vertebral bodies via an in-line approachthat is substantially perpendicular to the anterior wall of the standalone cage.

U.S. Pat. No. 6,551,322 (Lieberman) discloses using a double helix as afusion cage.

A number of investigators have attempted to solve the above-statedproblem by in-line inserting a helical element into the in-situ cage.See for example, US 2002-0177898 (Crozet) U.S. Pat. No. 7,056,341; U.S.Pat. No. 6,210,442 (Wing); WO 00/16711 (Meriwether), and WO2010/028056(Synthes). However, each of these solutions largely involves passing thehelical element substantially through the graft-containing region of thecage, thereby reducing the space in the cage available for the criticalbone graft.

Although the helix of FIG. 23 of Crozet does not enter the graft space,the height of Crozet's helix (a screw) would need to be only slightlygreater than the height of the cage in order to avoid the hittinganterior lips of the adjacent vertebral bodies during insertion. Also,screw backout could be an issue.

Moreover, in each of these prior art solutions, the longitudinal axis ofthe helix is substantially parallel to the upper and lower surfaces ofthe cage.

SUMMARY OF THE INVENTION

A number of solutions to the above stated problems have been developed.Stand alone fusion cage assemblies that can be secured to the adjacentvertebral bodies via an in-line approach that is substantiallyperpendicular to the anterior wall of the stand alone cage (orsubstantially parallel to the cage insertion direction).

DESCRIPTION OF THE FIGURES

FIG. 1A is a perspective view of a hollow bellows having a helicalthreadform.

FIG. 1B is an axial cross-sectional view of a hollow bellows having ahelical threadform.

FIG. 1C discloses a bellows screw comprising a proximal head having ahollow center, an intermediate bellows section and a bored distal tip.

FIG. 2A is a side view of the installation of a conventional stand alonecage.

FIGS. 2B and 3 are side views of the installation of a stand alone cagefixed with a hollow bellows screw.

FIG. 4 is a side view of an implanted stand alone cage fixed with ahollow bellows screw.

FIG. 5 discloses a preferred screw.

FIGS. 6A-B disclose cross-sectional views of a first set of preferredheads.

FIGS. 7A-B disclose a second set of preferred heads.

FIG. 8 discloses a preferred bellows.

FIGS. 9A-B disclose cross sections of preferred tips.

FIG. 10 discloses a preferred embodiment with mechanical joints.

FIGS. 11A-B disclose a preferred method of inserting the screw.

FIG. 12 is a perspective view of a conventional spherical hex.

FIGS. 13-14 are perspective and cross-sectional views of ascrew-inserter assembly having a spherical hex joint.

FIGS. 15-16 are perspective and cross-sectional views of ascrew-inserter assembly having a projection slidably received in a slot.

FIGS. 17A-B and 18 are side, perspective and cross-sectional views of ascrew-inserter assembly having a capped projection slidably received ina slot.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, a “helix” includes structuresthat have less than a full period of a helix.

Bellows Embodiment

In preferred embodiments, the helix is present in the form of a helicalbellows 1, as shown in FIG. 1A.

A traditional metal bellows is a metallic flexible compressible hollowcylindrical structure; the walls are thin and corrugated. The ridges andfurrows of the walls facilitate the flexibility of the structure, and aslong as the material yield strength is not exceeded the bellow willreturn to its original form after deformation. Typically thecorrugations run parallel to each other and perpendicular to the axis ofthe cylinder. Metal bellows can be manufactured through forming,welding, or electroforming. Forming bellows involves rolling, deepdrawing, or hydroforming. Formed bellows typically have thick wallsresulting in reduced flexibility when compared to welded orelectroforming. Material choices for formed bellows are restricted tometals with high ductility. Welded bellows are made by edge welding astack of formed rings. Weld location alternating between the inside andoutside edge creates the bellows. Wall thickness is thin and flexibilityis high. Electroformed metal bellows are produced by electrodeposition,where a thin layer of metal is electrically deposited on to an aluminumform called a mandrel. The mandrel is precision machined and comprisesthe geometry of the finished form. Once the desired thickness ofmaterial (typically nickel, copper, gold, silver, or a combination oftheir alloys) is deposited on the mandrel the aluminum is chemicallyremoved leaving behind the finished bellows.

By changing the corrugations from parallel to a helix, a flexible threadform can be produced similar to a metal bellows. The hollow center wouldallow it to slide over a rod-like structure, and the flexibility wouldenable it to conform if the rod was curved. A drive feature at one enddelivers the means for rotation, and the helical threads provide amethod to advance the flexible screw.

FIG. 1B shows a cross section of the helical bellows-type flexible screw3.

FIG. 1C discloses a bellows screw comprising a proximal head 5 having ahollow center 6, an intermediate bellows section 7 and a bored distaltip 9.

The flexible screw could be used to anchor a cage into a vertebral bodythrough a small access site. A traditional straight screw would requirea large opening if a high angle screw trajectory was desired. Theflexible screw trajectory could be curved, resulting in a much smalleropening for high angle screw placement.

FIGS. 2A and 2B compare the trajectories of the screws 7.

Now referring to FIG. 3, once the cage is placed between the vertebralbodies a curved stylet 9 is used to create the desired high angletrajectory of the screw. The flexible screw 7 is then placed over thestylet and a cannulated driver 11 engaged with the external drivefeature is used to rotate the flexible screw. Once the screw 7 is fullyseated the stylet can be removed, see FIG. 4. The head of the screwkeeps the cage from dislodging.

Therefore, in accordance with the present invention, there is providedan intervertebral assembly comprising:

-   -   a) an intervertebral fusion device having an anterior wall, a        posterior wall, and first and second side walls connecting the        anterior and posterior walls, an upper bearing surface and a        lower bearing surface, the anterior wall having a height and an        anterior surface, and a first threaded throughhole extending        into the anterior wall from the anterior surface,    -   b) a fixation device comprising a proximal driving head and a        distal flexible bellows having a helical threadform disposed in        the first throughhole, wherein the helical threadform threadably        mates with the first threaded throughhole.

Preferably, the fixation device bellows has a throughhole extendingthrough the longitudinal axis thereof.

In general, and now referring to FIG. 5, the device is a screw 7 thatcomprises three sections: a proximal head 13, an intermediate flexiblethreaded hollow bellows section 15, and a distal tip 17.

In some embodiments, and now referring to FIGS. 6A-B, the head containsan internal drive feature 18 that mates with an instrument to transfertorque to the screw causing it to advance via the screw threads. Theinternal drive feature could be a hex, hexlobe, or square. In someembodiments, there is also an axial through-hole in the head to allowthe screw to follow a guide wire. The hole could be cylindrical orfrustoconical: If the hole is cylindrical 19, its diameter would need tobe of sufficient size to allow the screw head to pass over a curvedguide wire. If the hole is frustoconical 21, the narrow part of thefrustocone would keep the screw head centered but the wide part of thefrustocone would allow the head to pass over a curve in the guide wire.

Now referring to FIG. 7A, the screws used to fix the stand-alone fusioncage in place must fasten to the cage to ensure the cage does notmigrate and the screws do not back out. One method of securing thescrews is to use a tapered threaded head 23. Now referring to FIG. 7B,another embodiment uses a clip that mates with a peripheral groove 25 inthe head. Both of these means are established locking mechanisms used insecuring the bone screws to plates and/or cages.

Now referring to FIG. 8, the bellows section 27 of the screw is flexibleand hollow, thereby allowing the screw to slide over and follow the pathof a guide wire. Helical threads formed by the bellows enables the screwto axially advance when torque is applied. The outer diameter of thethreads should be less than the diameter of the screw head, as thisenables the screw threads to pass through the hole in the cage or platewhile the screw head cannot.

Now referring to FIG. 9A-B, the distal tip of the device is used toguide the screw along the guide wire and incorporates a similarcylindrical 29 or frustoconical 31 axial through-hole as the head. Alsopreferably incorporated on the tip are tapered threads of the same pitchas the bellows threads. The threads of the tip have an axial cut,thereby creating a flute as a cutting edge.

Now referring to FIG. 10, the proximal head 33, intermediate threadedbellows 35, and distal tip 37 can all be made separately and joinedtogether at first 39 and second 41 joints to create the screw. Methodsof joining could include welding, brazing, gluing, or mechanicalswaging.

The curved guide wire should be of a material that is stiff and strongenough to maintain its curve while being inserted through the cage orplate and into the vertebral body. Pre-curved wires would be preferredto ensure the screw can flex appropriately to conform to the guide wire.

Now referring to FIGS. 11A-B, a sleeve 43 can be used over the guidewire 45 to create a slightly larger path in the vertebral body. Once thesleeve is removed from the guide wire, the extra space left by thesleeve will assist with screw insertion.

A centering guide 47 can be inserted into a hole 48 in a plate or cage49 to ensure the guide wire is located centrally in the screw.

Spherical Hex Embodiment

In another embodiment, and now referring to FIGS. 12-14, there isprovided an inserter-device assembly wherein the driver head of thefixation device incorporates a spherical hex. For the purposes of thepresent invention, a “spherical hex” shape is a spherical surface havingsix equally-spaced flats 51 around a perimeter of the spherical surface53. When a traditional ball and socket joint is modified so that thespherical hex replaces the ball and a hexagonal socket replaces thespherical socket, the resultant design allows the transmission of torquebetween the two components. The proximal end of the threaded “screw”component can be either a male (head) or female (socket) end. Aspring/ball plunger or other means can be used to apply friction to thespherical hex head to keep the two components rigidly engaged. Onceassembled, the mouth of the hex socket is deformed to close around thebase of the spherical hex head to ensure the two components do notdisassemble. The size and shape of the geometry of the neck at the baseof the spherical hex head helps determine how much angulation isallowed. Although a hex has been described, any cross section (square,pentagon, octagon, etc.) with flats to engage a corresponding socketwhich allows the transmission of torque is envisioned.

Therefore, in accordance with the present invention, there is providedan intervertebral assembly comprising:

-   -   a) an intervertebral fusion device having an anterior wall, a        posterior wall, and first and second side walls connecting the        anterior and posterior walls, an upper bearing surface and a        lower bearing surface, the anterior wall having a height and an        anterior surface, and a first threaded throughhole extending        into the anterior wall from the anterior surface,    -   b) a fixation device comprising a proximal driving head and a        distal shaft having a threadform disposed in the first        throughhole, wherein the threadform threadably mates with the        first threaded throughhole, and    -   c) an inserter having a distal endportion that mates with the        driving head,        wherein the distal endportion of the inserter and the driving        head form a joint 55 comprising i) a head element 57 comprising        a spherical surface having a plurality of flats spaced        equidistantly along a perimeter of the spherical surface,        and ii) a socket element 59 comprising a mating surface mating        with the plurality of flats 61.

Preferably, the distal endportion of the inserter and the driving headform a spherical hex joint.

Also in accordance with the present invention, there is provided anintervertebral assembly comprising:

-   -   a) an intervertebral fusion device having an anterior wall, a        posterior wall, and first and second side walls connecting the        anterior and posterior walls, an upper bearing surface and a        lower bearing surface, the anterior wall having a height and an        anterior surface, and a first threaded throughhole extending        into the anterior wall from the anterior surface,    -   b) a fixation device comprising a driving head and a distal        shaft having a longitudinal axis and a threadform disposed in        the first throughhole, wherein the driving head comprises a        spherical surface having a plurality of flats spaced        equidistantly along a perimeter of the spherical surface,        wherein the perimeter is substantially perpendicular to the        longitudinal axis of the shaft, and        wherein the threadform threadably mates with the first threaded        throughhole.

In another embodiment, a cable is attached between the distal screw endand a proximal socket. The ends of the cable can be welded in place torigidly attach the socket to the screw. The cable provides flexibilityyet allows the transmission of torque between the socket and screw.

Slidable Gear Joint Embodiment

In another embodiment, and now referring to FIGS. 15-18, there is i) abone anchor 71 (or screw) having a shaft defining a first longitudinalaxis and helical threads 73 extending from the shaft, and ii) a driver75 also having a second longitudinal axis, wherein the proximalendportion of the bone anchor slidingly interfaces with the distalendportion of the driver in a manner whereby the longitudinal axis ofthe driver and the longitudinal axis of the screw are not necessarilycollinear or parallel, but rather form an oblique angle. The distalendportion of the driver and the proximal endportion of the bone anchoreach contains a mating gear element 77, 79 so that rotation of thedriver resulting in driving the screw through the transmission of torquethrough the gear. The driver and screw being positioned at an obliqueangle allows the driving to proceed without the requirement of in-lineforce transmission. In some embodiments, the torque is transmitted by abeveled gear mechanism having teeth on the screw and teeth on the drive.Rotation of the driver about its longitudinal axis will cause rotationof the screw about its longitudinal axis.

The mechanism for keeping the driver and screw engaged (while the driverdrives the screw) could take any of several forms. The proximalendportion of the Screw can have a rotatable post which extendsproximally from the screw head along the screw axis, wherein the post isretained by and rotatable along with the body of the screw. The postpreferably also has a transverse opening 81 (such as a slot or a hole)to accept a distal portion of the driver tip.

Therefore, in accordance with the present invention, there is providedan intervertebral assembly comprising:

-   -   a) an intervertebral fusion device having an anterior wall, a        posterior wall, and first and second side walls connecting the        anterior and posterior walls, an upper bearing surface and a        lower bearing surface, the anterior wall having a height and an        anterior surface, and a first threaded throughhole extending        into the anterior wall from the anterior surface,    -   b) a fixation device comprising i) an elongate proximal        projection defining a longitudinal axis and having a throughhole        extending transversely therethrough, ii) an intermediate driving        head having a first geared surface facing proximally, and iii) a        distal shaft having a threadform disposed in the first        throughhole, wherein the threadform threadably mates with the        first threaded throughhole, and    -   c) an inserter having i) an elongate distal endportion, ii) a        proximal endportion having a handle, and iii) an intermediate        portion therebetween having a second geared surface that mates        with the first geared surface of the proximal driving head,

wherein the elongate distal endportion of the inserter is slidablyreceived in the throughhole of the proximal projection of the fixationdevice.

Preferably, the proximal projection of the fixation device isdetachable.

In a first preferred embodiment, the thoughhole of the proximalprojection of the fixation device defines:

-   -   a) a proximal inner surface disposed at an angle to the        longitudinal axis of the elongate proximal projection, wherein        the angle us between about 30 and 60 degrees, and    -   b) a distal inner surface disposed substantially perpendicularly        to the longitudinal axis of the elongate proximal projection,

In a second preferred embodiment the elongate distal endportion of theinserter has a distal head thereon.

We claim:
 1. A method of treating a spine, comprising the steps of: i)inserting into a disc space an intervertebral fusion device having ananterior wall, a posterior wall, and first and second side wallsconnecting the anterior and posterior walls, an upper bearing surfaceand a lower bearing surface, the anterior wall having a height and ananterior surface, and a first threaded throughhole extending into theanterior wall from the anterior surface, ii) disposing into the firstthreaded throughole a fixation device comprising a proximal head and anintermediate flexible bellows having a helical threadform, wherein thehelical threadform threadably mates with the first threaded throughhole.2. The assembly of claim 1 wherein the proximal head of the devicecomprises an internal drive feature.
 3. The assembly of claim 1 whereinthe proximal head of the device comprises an axial through hole.
 4. Theassembly of claim 1 wherein the axial through hole is cylindrical. 5.The assembly of claim 1 wherein the axial through hole is frustoconical.6. The assembly of claim 1 wherein the proximal head of the device istapered and threaded.
 7. The assembly of claim 1 wherein the proximalhead of the device has a peripheral groove.
 8. The assembly of claim 1further comprising c) a guidewire extending through the fixation device.9. The assembly of claim 8 further comprising d) a sleeve, wherein theguide wire extends through the sleeve.
 10. The assembly of claim 9further comprising e) a centering guide disposed in a hole in theintervertebral fusion device, wherein the sleeve extends through thecentering guide.
 11. The assembly of claim 1 wherein the device furthercomprises a distal tip.
 12. The assembly of claim 11 wherein the distaltip has a tapered thread.
 13. The assembly of claim 11 wherein the head,bellow and tip are independent elements joined by first and secondjoints.